Switching your plan does not turn on the GPS.
FIrst thing is to make sure you have the latest Sprint ROM, if not get it here:
http://www.htc.com/us/FAQ_Detail.aspx?p_id=75&act=sd
Then follow the instructions as per TalynOne's fantastic thread on 'priming your gps' for use with your gps software flavour. This stickied in the Titan thread, or can be found here:
http://forum.ppcgeeks.com/showthread.php?t=17032.
Once that's all settled,
gps - check
navigation software - check
check out astronusX fantastic little app that ties it all together on xda:
http://forum.xda-developers.com/showthread.php?t=364199
Hope that helps